Ribonucleases are enzymes that catalyze the degradation of RNA. A well studied ribonuclease is bovine pancreatic ribonuclease A (RNase A), the putative biological function of which is to break down the large amount of RNA that accumulates in the ruminant gut. The RNase A superfamily is a group of RNase enzymes classified as similar to RNase A which possess a number of interesting biological properties including antiproliferative, cytotoxic, embryotoxic, aspermatogenic, and antitumoral activities. One member of this family is a homolog of RNase A, originally isolated from oocytes and early embryos of the Northern leopard frog Rana pipiens. 
The frog (Rana pipiens) ribonuclease, when placed in a human cell, is not strongly inhibited by RI and its RNase activity destroys cellular RNA and kills the target cell. The anti-tumor properties, both in vitro and in vivo, of the frog ribonuclease are described and claimed in U.S. Pat. No. 5,559,212. This ribonuclease molecule is now known as Onconase® (ONC). The property of degrading RNA is essential to the cytotoxicity of ONC. ONC is currently being evaluated as a cancer therapeutic in clinical trials.
A significant limitation on the suitability of ONC as a chemotherapeutic is dose-limiting renal toxicity. ONC is retained in the kidney at concentrations much greater than mammalian members of the RNase superfamily. There may also be allergenic issues with ONC, since mice produce antibodies against ONC but not against RNase A, with which ONC shares about 30% of its amino acids. This suggests that other members of the RNase family may also be suitable candidates for evaluation as clinical therapeutics if they can be imbued with the cytotoxic properties similar to ONC.
In mammals, levels of RNase activity are controlled by a ribonuclease inhibitor (RI), which is a 50-kDa protein found in the cytosol of all mammalian cells. RI is a member of a leucine rich family of proteins and is composed of 15 alternating repeats arranged symmetrically in a horseshoe shaped molecule. RI has a large number of cysteine residues (32 in human RI) which means that it can only keep its shape and function in a reducing environment like the cytosol. RI acts to bind to members of the RNase superfamily, one RI to one molecule of RNase, and when so bound, RI completely inhibits the catalytic activity of the ribonuclease by steric blockage of the active site of the enzyme. The binding of RI to RNase is a very tight one, having a very high binding affinity.
Some RNase superfamily members, notably ONC and bovine seminal ribonuclease, possess the native ability to evade RI. The trait of evasion of RI is primarily responsible for the cytotoxicity of ONC and bovine seminal ribonuclease. It has also been found that RNase superfamily members, which are not natively cytotoxic, can be made cytotoxic by modifying their amino acid constituents, so as to inhibit binding to RI.
Using the three dimensional structure of the porcine RI (pRI)-RNase A complex, RNase A was engineered to be more toxic to human leukemic cells in vitro than ONC. Disruption of the RI•RNase A interface was accomplished by designing RNase A variants with amino acid substitutions that disrupted complementarity regions at the pRI•RNase A interface. These amino acid substitutions targeted short range pRI•RNase A interactions by incorporating sterically disruptive amino acids or removing hydrogen bonds. This method is described in U.S. Pat. No. 5,840,296, incorporated by reference herein in its entirety. Analogous complementarity regions were applied to bovine seminal ribonuclease (BS-RNase, 87% sequence similarity) a close homologue of RNase A. However, a BS-RNase variant with mutations at the same complementarity regions was less cytotoxic than ONC or the most cytotoxic RNase A variant (D38R/R39D/N67R/G88R RNase A). This strategy did not result in the level of cytotoxicity predicted for BS-RNase.
Furthermore, most of the work done so far in the creation of RNase A variants has been done with bovine RNase A. However, the sequence and structure of bovine RNase A (SEQ ID NO:1, GenBank Accession No. AAA72757) differs from human pancreatic ribonuclease 1 (RNase 1) (SEQ ID NO: 2, GenBank Accession No. CAG29314, incorporated by reference herein in its entirety). RNase A and its homolog, RNase 1 share about 70% sequence identity of their amino acid sequences. While the bovine protein may prove out to be acceptable for use in human therapy, a conservative approach might be to utilize a variant of a human ribonuclease, on the theory that use of a human protein might minimize cross-species antigenic problems. Accordingly, it is desirable to design variants of human ribonucleases that may be more cytotoxic and effective for therapeutic, diagnostic or research use.